Electrical fusible element having three portions of different cross-sections



N 1966 J. FEENAN ETAL 3, 88,968

ELECTRICAL FUSIBLE ELEMENT HAVING THREE PORTIONS OF DIFFERENT CROSS-SECTIONS Filed Jan. 30, 1964 United States Patent 3 288,968 ELECTRICAL FUSIBLE ELEMENT HAVING THREE PORTIONS OF DIFFERENT CROSS-SECTIUNS John Feenan, Eric Jacks, and John Eric Murphy, all of Liverpool, England, assignors to The English Electric Company Limited, London, England, a British Cornan p y Filed Jan. 30, 1964, Ser. No. 341,267 Claims priority, application Great Britain, Feb. 8, 1963, 5,145/ 63 1 Claim. (Cl. 200135) The invention relates to electrical fuse-elements and is particularly applicable to fuse-elements for use in cartridge fuse links of the type in which a fuse-element is supported within a tubular insulating container by two conductive members secured to the container one at each end thereof.

According to the invention, a fuse-element includes a strip of fusible metal having at least two full-section portions and a neck portion separating the two full-section portions, the thickness and width of the neck portion being less than the thickness and width respectively of the fullsection portions and the width of the neck portion being substantially constant.

According to a preferred feature of the invention, the ratio of the cross-sectional area of each of the full-section portions in a plane perpendicular to the length of the strip to the cross-sectional area of the neck portion in a similar plane is of the order of :1.

Preferably, the thickness of the neck portion is as low as possible commensurate with maintenance of adequate mechanical strength.

A preferred form of fuse-element embodying the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of the fuse-element; and

FIG. 2 is a fragmentary view, to an enlarged scale, of part of the fuse-element.

The fuse-element comprises a strip 10 of silver which is provided with a series of neck portions 11 joining fullsection portions such as those indicated at 12. The neck portions 11 are disposed centrally relative to the width of the strip 10 and have a thickness x (FIG. 2) which is less than the thickness y of the full-section portions 12. The cross-sectional area of each neck portion 11 in the plane perpendicular to the length of the strip 10 is approximately the cross-sectional area of each full-section portion 12.

The fuse-element includes a thickened portion 13 for reducing the overall resistance of the fuse. Such a portion may be positioned at either or both ends of the element and/ or in the middle of the element.

A method of forming a neck portion is to produce a laterally-extending groove in the strip, defining the thickness of the neck portion, by a process of rolling or milling, for example, and then to form slots extending inwardly from the ends of the groove so as to define the lateral width of the neck portion.

It has been found that a ratio between the cross-sectional areas of the full-section portions and the neck portion of the order of 10:1 (that is, ranging from about 8:1 to about 12:1), as in this case, is an important factor in producing a fuse with a high current rating within limited dimensions and with a fast fusing action. It has also been found that the proportions of the neck portion affect the distribution of current in the full-section portions and hence the overall effective resistance of the fuse element which should be as low as possible. The best current dis- "ice tribution in the full-section portions and hence the lowest overall effective resistance) is obtained by reducing the thickness, rather than the width, of the neck portion: ideally the neck portion would consist of a groove extending across the whole width of the strip. However, the desired ratio of cross-sectional areas between the full-section portions and the neck portion cannot in many cases be obtained in this way without so reducing the thickness of the strip at the groove as to make the strip unmanageable mechanically. Therefore, by providing, in a fuse element according to the invention, a neck-portion having both a reduced width and a reduced thickness, a compromise can be made between the conflicting requirements of mechanical strength and good current distribution and at the same time the desired ratio of cross-sectional areas can be achieved. In a test illustrating this, a first fuseelement, three inches long was provided, with a series of six neck portions by simply forming slots in the sides of the strip so that the lateral width of the strip was reduced to of the full width. The measured resistance was 750 microhms. A second fuse-element of the same length had the thickness of the neck portions reduced from .010 inch to 0.003 inch enabling the width of the neck' portions to be 3 /3 times wider than in the first fuse element for the same cross-sectional area. The measured resistance of the second element was 600 microhms.

Desirably, the required cross-sectional area of the neckportion should be spread over as much of the width of the strip as possible commensurate with good mechanical strength: this also provides the most effective heat dissipation because, for a given cross-section, the largest possible surface area is available for heat transfer purposes.

What we claim as our invention and desire to secure by Letters Patent is:

A fuse element comprising a strip of silver substantially rectangular in cross-section throughout its length, said strip having a plurality of first portions,

a plurality of second portions, each said first portion being located between, and integral with, said second portions, and the width and thickness of each first portion being less than the width and thickness, respectively, of each second portion by an amount such that the ratio of the cross-sectional area of each second portion to each first portion is between about 8:1 to about 12:1, and

a third portion integral with at least one second portion, said third portion having a thickness greater than, and a width equal to, said second portion, the width of each said first, second and third portions being substantially constant throughout their length.

References Cited by the Examiner UNITED STATES PATENTS 1,124,133 1/1915 Hershey 200-131 1,426,827 8/ 1922 Eustice 29155.5 2,157,906 5/1939 Lohausen 200-1-35 2,157,907 5/1939 Lohausen 200-431 X 2,471,176 5/1949 Von Hoorn 200-135 2,665,348 1/1954 Kozacka 200- 2,777,063 1/ 1954 Kozacka 200120 2,780,696 2/ 1957 Lear 200-435 BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner. 

